The present invention is directed to hydraulic control systems, and pertains more particularly to a hydraulic control system having a plurality of variable displacement pumps and means for automatically adjusting the displacement of the pumps to changes in engine output.
Many hydraulically actuated implements, such as a hydraulic excavator, normally employ a plurality of variable displacement pumps for supplying the high-pressure hydraulic fluid necessary to power the many motors employed in manipulating the excavator bucket and linkage. Reference is made to U.S. Pat. No. 3,841,795, issued Oct. 15, 1974 to Ferre et al., assigned to the assignee hereof, for background material pertinent to the subject invention. The above patent was developed to overcome certain problems of the prior art and functions very satisfactorily so long as a single predetermined engine speed or engine normal operating speed is selected and maintained.
Earth-moving machines, such as the hydraulic excavator, are normally designed to have a predetermined normal operating speed of the engine during normal operation of the machine. With such machines the engine throttle is normally set at a predetermined position when the machine is in normal operation.
Frequently, it is desirable to operate the vehicle at a lower or higher engine speed than the normal operating speed. This becomes a problem when the hydraulic system is equipped with a control system such as that of U.S. Pat. No. 3,841,795 which is operative to maintain a predetermined output of the hydraulic system to match the output of the engine. With such a system, as soon as the engine speed is reduced, the differential pressure across the underspeed valve orifices would signal the pumps of the system to stroke to a lower displacement as they would react as though the engine was running slower due to lugging such as from a load. Thus, one would be unable with that system to operate the vehicle and/or its implements with the engine at a lower speed and at the same time obtain full pump flow from the pumps.
A system for providing dual speeds of a system is disclosed in application Ser. No. 583,366, filed June 4, 1975, entitled Part Throttle Control - Pump Override, now U.S. Pat. No. 3,963,378 and assigned to the assignee hereof. With such a system, however, it has been found that because of the sensitivity of the underspeed valve, the pressure setting across the valve for the selected operating conditions are critical. Thus, for a selected engine throttle setting the proper pilot control pressure must exist for proper control of the underspeed valve.